1. Field of the Invention
The invention relates to a mobile unit's position measurement apparatus and a mobile unit's position measurement method for measuring the position of a mobile unit on the basis of signals from satellites.
2. Description of the Related Art
Apparatuses that measure the position of a mobile unit on the basis of signals from satellites have been widely used. A problem of these apparatuses is the so-called multipath phenomenon. This is a phenomenon in which when a receiver receives a signal from a satellite that is reflected from a building or the like, the acquired distance (pseudo-distance) between the receiver and the satellite deviates from the actual distance therebetween, so that the present position of the apparatus is falsely recognized.
In conjunction with this problem, an invention has been disclosed which relates to a method in which the positioning computation is performed through the use of only signals from satellites with respect to which a difference between an observed amount of Doppler shift and an estimated amount of Doppler shift that is calculated by the so-called Inertial Navigation System (INS) on the basis of an output of a gyro-sensor or a vehicle speed sensor is smaller than a threshold value (see, for example, Japanese Patent Application Publication No. 2001.264409 (JP-A-2001-264409)).
However, in this related-art method, it may happen that no satellite is selected if a relatively large error occurs in the position or velocity of a mobile unit which is calculated on the basis of an output of a gyro-sensor or a vehicle speed sensor. Therefore, in some cases, the position of a mobile unit cannot be continuously and accurately measured.